1. Field of the Invention
The present invention relates to a coating solution composition for forming a glass gel thin film comprising a metal alkoxide and a coloring material, a color glass gel filter, and a display device employing the glass gel filter.
2. Description of the Related Art
A filter having selective light-absorbing properties is formed on the outer surface of a conventional display device to improve its contrast characteristics. A glass-film is generally used in this filter. In order to obtain desired filter characteristics of this glass film, a coloring material having appropriate light-absorbing properties can be contained in the glass film. An organic coloring material is preferable as coloring material to obtain desired filter characteristics since many kinds of organic coloring materials are available. However, an inorganic coloring material can also be used.
A sol-gel method is generally used as a method of forming a glass film. According to this method, a solution mixture of a metal alkoxide, water, an acid, a coloring material and an alcohol, i.e., a coating solution composition, is hydrolyzed and polycondensed to obtain a gel film. In a general sol-gel method, the coating solution composition is sintered at a temperature of about 1,000.degree. C. However, an organic coloring material is decomposed at this temperature. To the contrary, a means for sintering a coating film containing a coloring material at a low temperature to form a glass thin film having selective light-absorbing properties is disclosed in Published Unexamined Japanese Patent Application No. 1-320742. In this Published Unexamined Japanese Patent Application No. 1-320742, a dye is used as a coloring material since the body color does not change by external light and desired light-absorbing filter characteristics can be obtained. As a result, practical selective light-absorbing properties can be obtained.
A glass thin film obtained by sintering at 500.degree. C. or less in the sol-gel method has many pores and is highly porous. When a dye or organic pigment capable of obtaining a sharp color is used as a coloring material, a maximum sintering temperature of a coating solution composition should be about 300.degree. C. or less due to heat resistance. For this reason, the resultant glass film tends to be porous. The glass film obtained by low-temperature sintering cannot be a sufficiently dense film. Many pores exist as defects in the film are numerous, thus resulting in a porous glass gel-like film (to be referred to as a glass gel film hereinafter).
As a result, particularly when a dye is used as the coloring material, the resulting product has a problem of water resistance. That is, water or chemicals permeate through the film surface to elute the coloring material. In addition, since a coloring material having a small particle size is used to improve transparency of a glass gel film, oxidation and discoloration of the coloring material by oxygen, ultraviolet rays, and the like cannot be perfectly prevented. This glass gel thin film tends to allow permeation of oxygen. In addition, when a glass gel film has defects in its skeleton structure, its mechanical strength tends to decrease.
Under these circumstances, there has been proposed a means for vitrifying a material by high-temperature sintering using an inorganic coloring having material excellent heat resistance in place of using an organic coloring material having poor heat resistance, thereby eliminating the defects of the glass gel thin film obtained by low-temperature sintering and hence obtaining a high mechanical strength. For this purpose, however, the material must be sintered at a temperature of 500.degree. C. or more for a long period of time. It is very costly to perform high-temperature sintering for a long period of time. In particular, it is very expensive to perform such high-temperature sintering of electronic devices such as a display device. It is also difficult to obtain desired filter characteristics by using only inorganic coloring material due to less kinds of inorganic coloring material compared with an organic coloring material.
A glass film having filter characteristics has been formed by a conventional sol-gel method. In this method, since a low sintering temperature is set in consideration of heat resistance of a coloring material to be used, the resultant glass gel film is porous and has poor water resistance. The particle size of the coloring material must be small to improve transparency of the film. The conventional glass film thus poses a problem on the resistance to ultraviolet rays. In addition, the strength of the film on the filter is not sufficient.